1. Field of the Invention
This invention relates to a picture-in-picture television function (hereinafter referred to as a P-in-P function) of a double-scanning television receiver which performs sequential scanning conversion.
2. Description of the Related Art
Recent television receivers perform scanning line interpolation and sequential scanning line conversion to improve the picture quality, as done in the improved-definition television (IDTV). When such an improved television receiver is provided with the picture-in-picture function, it is essential that a sub-picture should also have the improved picture quality.
The conventional P-in-P function will be explained with reference to FIGS. 1 through 3. FIG. 1 is a block diagram of the P-in-P function based on a one-field memory of the interlaced scanning television receiver. (JP-A-55-39472 and 61-166279)
In FIG. 1, 61 designates an input terminal for the video signal of a main picture, 62 an input terminal for the video signal of a sub-picture, and 63 a one-field memory for storing the input video signal of a sub-picture for one field and it is capable of writing in and reading out the signal in every horizontal scanning period (one line). 64 designates a circuit for modifying the interlacing of the sub-picture, 65 a circuit for inserting the sub-picture in the main picture, and 66 an output terminal for the video signal of the composite main and sub picture.
The operation of the television receiver having the P-in-P function based on the one-field memory arranged as described above will be explained with reference to FIG. 2. FIG. 2 is a timing chart for the arrangement shown in FIG. 1, and this is a case where a sub-picture has horizontal and vertical dimensions that are 1/3 those of an associated main picture. In FIG. 2, I denotes the video signal of the sub-picture, II denotes the video signal of the main picture, and III is intended to explain the operation of the one-field memory 63, in which IV shows the write timing and V shows the read timing for the memory.
Writing in the field memory 63 of FIG. 1 is performed at every horizontal scanning line when a picture is vertically scaled down to half size, and at every three scanning lines when a picture is vertically scaled down to one-third size. Reading of the field memory takes place at a timing corresponding to the position of insertion in the main picture with a clock frequency high enough to meet the scale factor. In FIG. 2, the writing takes place in every third horizontal period, and the reading takes place three times as fast as the writing thereby to compress the sub-picture to have dimensions of 1/3 size. Since writing and reading take place sequentially in one horizontal period, the maximum time length for inserting a sub-picture in a main picture can not extend over an entire period, but, in case of NTSC, a 3/4 line period is sufficient (shown by hatching) in consideration of the horizontal flyback period and the like. The video signal of a main picture and that of a sub-picture are derived from completely independent transmission systems, and they are not synchronous with each other. Therefore, reading from the field memory 63 may overtake writing therein with high probability, and in this case the content of the previous field will be read out during writing. This action causes the sub-picture to have opposite interlacing, resulting in a non-smooth picture. The interlace correction circuit 64 in FIG. 1 is intended to correct the interlacing at the timing of a vertical sync pulse of the sub-picture, and it detects field switching in the midst of the sub-picture to modify the reading address so that correct interlacing takes place at that point.
FIG. 3 is a block diagram of a double-scanning P-in-P block which is intended for a double-scanning television receiver based on the sequential scanning conversion scheme shown in FIG. 1. 71 designates an input terminal for the video signal of the main picture, 72 the input terminal for the video signal of the sub-picture, 73 a scanning line interpolation circuit which produces interpolated data for double-scanning conversion, 74 and 75 field memories for the video signal of a sub-picture, each being capable of writing in and reading out the signal at every line, of which the field memory 74 stores the present lines and the field memory 75 stores the interpolated lines, 76 an interlace correction circuit which functions identically to the interlace correction circuit 64 in FIG. 1, 77 and 78 main and sub picture composing circuits, 79 a double-speed conversion circuit, and 80 an output terminal for the composite video signal. The sub-picture video signal is written in the two field memories 74 and 75, and both contents are read out simultaneously in synchronism with the main picture video signal, thereby accomplishing the P-in-P function. Because of a doubled number of lines for the sub-picture as compared with the conventional scheme, a high quality P-in-P picture can be realized. (Refer to "Digital Television with Enhanced Picture Quality", Toshiba Review, Vol. 42, No. 12, Dec. 1 1987)
The above-mentioned double-scanning P-in-P television receiver operates, for the main picture, to make the sequential scanning conversion through the interpolation based on a motion adaptive process, while, for the sub-picture, it operates to make sequential scanning conversion for the interlaced scanning video signal after it is subjected to line skipping thereby to be contracted, and therefore the television receiver has a problem of line flicker in the sub-picture.